Dr. Douglas Tucker, an associate professor at the UC San Francisco School of Medicine, said the images taken of Hirschfield’s brain over the past few weeks indicate structural and functional deficiencies in the amygdala, a cluster of neurons deep within the brain linked to emotional processing and impulse control.

“His brain is not normal,” said Tucker, called by Hirschfield’s defense attorneys to present a report written by Ruben Gur, a University of Pennsylvania neuropsychologist who was unavailable to testify himself.

“This is a clear-cut structural abnormality that is described by Dr. Gur as brain damage,” Tucker said. And while Hirschfield’s brain overall is normal in size, in the amygdala “there are very clearly pathological levels of tissue loss.”

The damage put Hirschfield at risk of developing antisocial behavior and diminished his ability to learn from punishment, Tucker added.

Tucker’s testimony came on the second day of the penalty-phase proceedings for Hirschfield, 63, who was convicted earlier this month of kidnapping and killing UC Davis sweethearts John Riggins and Sabrina Gonsalves on Dec. 20, 1980. Jurors now must decide whether Hirschfield should be sentenced to death or life in prison without the possibility of parole.

Defense attorneys, hoping to spare their client from execution, have said Hirschfield endured an abusive and “chaotic” childhood that mitigates his later criminal conduct.

Tucker told defense attorney Linda Parisi that Hirschfield’s brain-scan results are consistent with a neurodevelopmental disorder, meaning the damage dates back to his birth or early childhood and worsened as he aged. Brain trauma also may have played a role, Gur concluded in his report.

Under cross-examination by Deputy District Attorney Dawn Bladet, Tucker acknowledged he had only received a copy of Gur’s report the night before his testimony, though he said he previously discussed the case with Gur over the phone.

He also said the report relied solely upon the scans, and that neither he nor Gur interviewed Hirschfield, researched his background or reviewed any of his medical records.

A 1986 motorcycle accident and a severe beating delivered to him at the Sacramento County Jail in 2004 could have contributed to Hirschfield’s brain damage, Tucker said, though he noted the scans “most likely are representative of what his brain was like throughout his life.”

“Do I have 100 percent certainty about that? No,” he added. And when told Hirschfield may have stalked Riggins and Gonsalves for days before the murders, Tucker said, “I wouldn’t call that an impulsive crime.”

The defense later elicited testimony Thursday from three of Hirschfield’s childhood friends, all of whom attended grammar and high school with him in the rural town of Colusa.

There, Hirschfield strived to be part of the “in” crowd, but his family’s low socioeconomic status kept him from succeeding, the witnesses said.

“He had a very serious desire to be well-liked, to be popular,” said Ernest Hubbard Jr., a fellow Colusa High School student from the 1960s. Hirschfield lacked the “assets” for popularity, and “it troubled him greatly, emotional and otherwise.”

Teasing was common among kids who were “on the outs,” Hubbard said, but added that he never saw Hirschfield taunted himself.

Home life was no picnic either, said the onetime friends, who remembered visiting Hirschfield at a meager house with six children and no father figure in sight.

“Instead of hanging out with friends, he would go home, and the sense I got was he was responsible for his brothers and sisters,” said John Moran, another former schoolmate.

“He always had a job of some kind. He just had to work all the time,” added Steve Codorniz, whose family lived several blocks away from the Hirschfields. “I just never felt like it was a happy time for him.”

But Hirschfield also was highly intelligent, said Codorniz, who told an investigator that he “figured he would run NASA or end up in prison.”

Hirschfield’s future also came up in a long-ago discussion with Hubbard, who remembered once asking his classmate, then about 15 or 16 years old, what he planned to do with his life.

“He made a very profound statement,” Hubbard recalled. “He said, ‘I don’t know, but what I do know is that when I die, I want people to remember me.’ ”